Redesign join API and internals

[gene-bordegaray]

Summary

This PR is stacked on top of the root execution/readiness API redesign in #77. It reshapes the libcudf-rs join API and internals around smaller, explicit join paths.

It replaces the monolithic join implementation with focused modules for common selection logic, equi joins, hash joins, join indices, masks, and output gathering.

Stream API details

The reusable hash join can be converted into a streaming join with into_streaming_join(). Streaming probes record matched build indices as each probe batch runs, and unmatched_build_rows(...) finalizes left/full-style output by gathering build rows that were never matched.

Filtered streaming probes use the same builder-style API as regular probes: filter(predicate), optional condition_tables(...), optional payload overrides, and selected build/probe outputs. The filtered path computes equality candidates first, applies the AST predicate, records only surviving build matches, and appends unmatched probe rows for left probes.

The stream readiness model keeps hash join state, filtered build state, and join index vectors alive until dependent GPU work is ready.

Internal structure

• Split src/join.rs into src/join/{common,equi,hash,indices,masks,output}.rs.
• Localized join state next to the code that owns it.
• Kept output gathering and filtered-index argument structs only where they protect same-typed FFI arguments.
• Removed thin compatibility wrappers and stale join abstractions.
• Converted production join panics into CuDFError paths.

Validation

• git diff --cached --check
• cargo fmt --check
• cargo check --workspace
• cargo clippy -p libcudf-rs -- -D warnings
• cargo test -p libcudf-rs join:: -- --nocapture
• cargo test -p libcudf-datafusion physical::hash_join -- --test-threads=1 --nocapture
• cargo test -p libcudf-datafusion expr::ast -- --test-threads=1 --nocapture
• cargo doc -p libcudf-rs --no-deps